The size of internal impedance in batteries, in the case of large capacity batteries, is very small at 1 mΩ or less. Likewise, the voltage (VIs′) generated by internal impedance in battery cells is a small signal of a number of mV. The voltage is very small (one-some thousandths) compared with the cell post voltage of 1.0-15V, and it's mixed with much electromagnetic wave noise from the surrounding area. Therefore, there is a need to appropriately separate this signal from a battery cell voltage (VDC), remove the noise from the signal by means of an optimized design of the noise removal circuit like the disclosed band-pass filter aimed at amplifying the signal appropriately, and input accurate, high resolution impedance voltage signals into the A/D converter unit in the main processor unit (MPU).
Also, there are protective fuse contact resistance, connected line resistance and parasitic impedance components in the four-terminal network circuit that connects the above signals to the input of the measuring circuit, and there is likewise a parasitic impedance value within the measuring circuit as well. Thus, in case of measuring the voltage generated by internal impedance in batteries, because the internal impedance value is a very low signal, there is a need to work out the methods which are designed to eliminate effects of parasitic impedance such as voltage drop value due to the contact resistance in the 4-terminal network and measuring circuit, and the resistance in the cabling line.
The method presented here uses one high common mode voltage differential amplifier not only to enlarge the measuring range of the battery cell voltage and to enhance the resolution for precise measurement but also to couple the impedance voltage contained in the direct current components of the battery cell voltage with the coupling condenser, and then filters the noise through the band-pass filter, and makes the signal precise and high in resolution, and converts the signal into a digital signal by means of A/D converter, and computes it to obtain the value of impedance.